Photographic light-sensitive materials make use of proteins and, in particular, gelatin as binders. For example, gelatin is used as the main component of silver halide emulsion layers, protective layers, filter layers, intermediate layers, antihalation layers, backing layers and subbing layers of photographic light-sensitive materials.
It is known that the mechanical properties of gelatin-containing layers of photographic materials can be improved by addition of hardening agent. In fact, the photographic materials may be stored at elevated temperatures and humidity conditions or treated with various aqueous solutions having different pH's and temperatures, and gelatin layers which have not been treated with a hardening agent have poor water resistance, heat resistance and abrasion resistance.
Many compounds are known to be effective for increasing mechanical resistance of a gelatin layer by hardening. They include, for example, metal salts such as chromium, aluminum and zirconium salts; aldehydes such as formaldehyde and glutaraldehyde; halogenocarboxyaldehydes such as mucochloric acid; aziridine compounds such as those described in U.S. Pat. No. 3,017,280; epoxy compounds such as those described in U.S. Pat. No. 3,091,537; halogenotriazine compounds such as hydroxydichlorotriazine and aminodichlorotdazine; and compounds having vinylsulfonyl groups such as methylene-bis-vinylsulfone, divinylsulfone and methylene-bis-vinyl-sulfonamide.
A group of hardening agents for photographic gelatin-containing layers which is particularly interesting has been disclosed in U.S. Pat. No. 4,063,952. These hardening agents are carbamoyl pyridinium compounds in which the pyridine ring carries a sulfo or sulfoalkyl group. These compounds have a high water solubility, a fast hardening action for gelatin and low occurrence of post-hardening (post-hardening is the change of hardening degree caused by slow continued hardening of the gelatin). They belong to the group of the so-called "fast acting" hardeners, with which the light-sensitive photographic materials can be hardened to an optimum degree within a very short time.
Owing to this rapid action, said fast acting hardeners generally should only be added to the gelatin containing casting solutions shortly before casting because otherwise a premature reaction would take place which would rapidly and irreversibly affect the casting properties, in particular the viscosity of the casting solutions.
The interaction of hardeners with the gelatin of the protective layer of a silver halide photographic material is known as a critical point: some solutions have been proposed in the past for conventional hardeners, such as triazine type hardeners (see, for example, U.S. Pat. Nos. 3,749,573; 4,944,966 and JP 59-151,151). These hardeners are either added to a gelatin-containing solution before the coating process, or applied, as a special coating, to a gelatin-containing layer already on the support material. However, this technology is not sufficient to avoid coating defects for fast acting hardeners belonging to the class of carboxyl activating hardener. In fact, due to the presence of restricted flow areas in the feeding line and in the coater which can not be completely eliminated, and due to the thin film of the liquid stream which adheres to the wall of the feeding line and of the coater, some portions of the liquid stream is characterized by a longer permanence time before coating. If this fact does not generate criticality with conventional hardeners, hardeners characterized by a faster kinetic of reaction with gelatin, such as the carboxyl activating type hardener, result in the formation of microgels which are occasionally stripped out from the liquid stream and reach the coater, where they generate defects of various types. To avoid this problem, it is necessary to periodically stop the production and clean the feeding line and the coater, thus reducing productivity.
U.S. Pat. No. 4,942,068 discloses a way to fully exploit the good performances of fast acting hardeners; it involves the introduction of the hardener through a dedicated layer of specific gelatin content and using a V-shaped coater for curtain coating. The hardening layer is guided at-the negatively inclined surface of the V-shaped coater, at the opposite side of the sensitive layers; the hardening and the sensitive layer meet at the edge of the coater without generating coagulation. As a prior art statement, said patent discloses that the hardener has to be applied in a separate coating solution containing from 0 to 4% by weight, preferably from 0.5 to 1% by weight gelatin or gelatin derivatives either as an additional layer or as a part of a layer packet, in which the adjacent layers also have gelatin concentrations of at most only 4% by weight. Without this condition, it is necessary to apply the hardener in a second coating solution, after drying or with a separate passage through the machine.
EP 285,994 describes a photographic material with top coat containing alkali hydrolized gelatin having specified viscosity and swelling factor to avoid reticulation and soiling. The material is hardened with a fast acting hardener of the carbamoyl-pyridinium type. The gelatin used in the topcoat is alkali-hydrolized bone gelatin having viscosity higher than 20.0 milliPascal per seconds, when measured in 10 weight % aqueous solution at 40.degree. C.
U.S. Pat. No. 4,421,847 discloses a process for chain-lengthening of gelatin in which the gelatin is brought into contact with a hardening agent which can activate the carboxyl groups of the gelatin. The so obtained partially hardened gelatin has advantageous properties as for the production of photographic layers.
U.S. Pat. No. 4,865,940 describes a color photographic recording material having at least one layer of binder and a dispersion of hardened particles of a complex coacervate (packet emulsion) in this layer of binder, wherein said particles of complex coacervate contain at least one carboxylic and amino group-containing proteinaceous polymeric binder and are hardened with a carbamoyl-pyridinium type hardener. The dispersion of hardened particles can easily be rehomogenized after concentration.
U.S. Pat. No. 4,119,464 discloses a process by which photographic layers can be hardened with carboxyl activating hardener, without the disadvantage resulting from the use of the large quantities of hardeners normally required for a fast acting hardening reaction. Said process includes the step, before applying the fast acting hardener, of incorporating in the surface of a photographic layer which contains gelatin, a pre-hardener selected from the group consisting of complex forming salts of aluminum, chromium and zirconium.
U.S. Pat. No. 5,034,249 discloses proteinaceous binders, in particular gelatin layers used in photographic recording materials, hardened by means of a fast acting hardener by casting a hardening system composed of at least two layers over the layer of binder, the lower of these two layers contains the fast acting hardener while the upper layer, which may be applied together with or immediately after the lower layer, contains a protein-containing binder but no hardener. The hardened layers have improved surface properties, such as wet scratch resistance and antifriction properties,
U.S. Pat. No. 4,978,607 describes a photographic recording material which comprises at least one gelatin containing silver halide emulsion layer and at least one protective layer containing a gelatin derivative, the protective layer being further away from the layer support than each silver halide emulsion layer and 30 to 90% of the amino groups of the gelatin in the gelatin derivative being reacted with a monofunctional acid derivative, and which is hardened with a fast acting hardener. The photographic material can be produced at high speed and, hence, at high drying temperatures without any reticulation grain occurring during processing.
DE 3,836,945 describes a photographic material with outer hardening coat containing thickener which is inert towards fast acting hardener and little or no gelatin to reduce soiling during processing.
DE 3,714,600 discloses a photographic silver halide material with double protective coat, the lower protective coat containing acid-ashed gelatin and surfactant polyalkylene oxide to prevent reticulation and soiling, while the upper protective coat contains a fast acting hardener and an acid-ashed gelatin (isoelectric point pH 5.0) or an alkali-ashed gelatin (isoelectric point pH 7.0-9.0).
DE 3,914,947 describes a photographic silver halide material with outer hardening coat containing both sulfoethylcellulose which is inert to fast acting hardener and anionic surfactant to reduce soiling.
It could be desirable to use a type of gelatin and a class of hardeners which combined together do not significantly affects the physical-mechanical properties of the film allowing in the meanwhile to optimize the use of fast acting hardeners.